When shooting loads in the .357 and .44 mag in the 1200-1300 fps range with cast bullets you need to keep a couple things in mind. For max loads stay away from the faster powders like Unique, Herco, AA5 etc. Instead use 296,H110, 2400, AA9 or Blue Dot and you will have much better results. My best accuracy in a number of mag handguns has always been with 296/H110 followed closely by 2400 and Blue Dot. The slower powders seem to give a softer launch.
Size to your cylinder throats and use a good lube such as the standard NRA 50/50,or any of the White label lubes. Do these things and lead bullet magnum bliss will be yours!